Transcriptional repression has recently been recognized to play a critical role in the regulation of mammalian gene expression. The mammalian transcriptional repressor RBP (CBF1) is notable in that it bridges the complex cellular processes of gene regulation, signal transduction, and development. RBP represses activated transcription from the adenoviral gene encoding polypeptide IX (pIX) and from the cellular gene encoding the cytokine interleukin-6 (IL-6). This repression requires the RBP- specific DNA binding site in both cases. Strikingly, the natural site at which RBP binds is immediately adjacent to other transcription factors in both cases. While the adjacent factors are different in each case, RBP binding in close proximity to the adjacent factor is required for repression. Yet, pIX repression does not entail RBP-mediated occlusion of adjacent factor binding. RBP directly interacts with two coactivators, TFIIA and TFIID, to block their interaction with each other and thereby inhibit pIX transcription. The studies proposed here will precisely determine the molecular basis of IL-6 repression by RBP. The approach includes analyses performed in vivo and in vitro. RBP will be tested for possible activator occlusion and/or interaction with activators/coactivators in mediating IL-6 repression. If interaction is detected, the targeted protein domain and associated activity that is inhibited will be identified. RBP functional domain(s) required for pIX and IL-6 repression will be mapped and correlation with RBP-mediated repression determined. Requirements for repression will be examined with respect to activator specificity in the context of the position of the RBP site. The biological relevance of RBP in re-establishing basal IL-6 expression will be studied in vivo with respect to RBP expression and the presence of the RBP- specific DNA binding site. These studies will broaden the current understanding of transcriptional repression at the molecular level in eucaryotes, in general. As well, identifying the targets of RBP in repression will provide the basis for understanding the role of RBP during mammalian development.